It is desirable to sense motor current flowing in momentum wheel and reaction wheel control electronics for motor control purposes. However, some prior art sensing methods require exotic hardware such as Hall Effect devices. Other prior art sensing methods have high power consumption and often produce unwanted heat, reducing efficiency.
One prior art current sensor uses ferrite cores with Hall Effect devices inserted in the magnetic path. A core is placed around each lead of a three-phase motor. The current in the magnetic field produces a proportional flux in the core that produces a voltage in the Hall Effect device proportional to the flux and the motor current. There are several drawbacks with this prior art sensor. For example, Hall Effect devices often have poor repeatability and can be unstable. Also, during orbit, satellites are subjected to ionizing radiation. While basic Hall Effect devices, such as thresholded, digital flux detectors, have proven radiation performance, the linear current sensor Hall Effect devices required for this sensitive space application have not been developed.
Another prior art sensor is shown in FIG. 1. The circuit has two sensing resistors 1 and 2. The circuit also requires that diodes, 3 through 8 in FIG. 1, be included in the motor current path at all times. The voltage drop in a diode can range from 0.6 volts to 1.0 volts, and can be the largest voltage drop of any component in the motor driver. The result of this voltage drop is a significant reduction in efficiency of the motor driver. Not only does this method waste bus power on a satellite, but it also produces heat, which introduces an additional problem of dissipating the waste heat.
A simple, power-efficient motor current sensor that does not require Hall Effect current sensors and power wasting, heat producing, diodes is needed to improve the efficiency of motor current sensing on board spacecraft.